It is commonly known that ultraviolet radiation causes colorants to degrade. This is a particularly acute problem for articles exposed to solar radiation for extended periods of time such as articles placed outdoors. Color degradation occurs in articles colored with conventional colorants as well as articles colored with fluorescent colorants. However, this is a particularly acute problem for fluorescent articles. Fluorescent dyes degrade, often turning colorless more quickly than conventional colorants. The effective life of fluorescent materials exposed to daily solar radiation is short and is typically measured in terms of days or months. In contrast, the life of conventionally colored materials is usually substantially longer and is typically measured in years.
If increased visibility of an article is desired, the article is often colored with fluorescent colorants even though fluorescent colorants are less durable than conventional colorants. Fluorescent colors allow enhanced visibility because the visual contrast that the fluorescent colors create with the environment make the materials more conspicuous than ordinary nonfluorescent articles. An example of one industry which benefits from using fluorescent colorants is the traffic sign industry. Fluorescent colored traffic signs are effective at increasing the visibility of the signs which increases motorist safety. Even though fluorescent signs increase motorist safety, their use has been limited due to their poor color stability and the need to frequently replace them to maintain effective performance.
At ground level, solar radiation comprises electromagnetic radiation having wavelengths longer than about 290 nanometers, with the range from about 400 to about 700 nanometers typically considered the visible light range. Radiation having shorter wavelengths than visible light is believed to be damaging to both conventional and fluorescent colorants. Attempts to maintain color of a fluorescent article have included adding ultraviolet screens which selectively filter radiation below the 340 nm to 380 nm range. Japan Kokai No. 2-16042, Application No. 63-165914 (Koshiji, et al.) discloses fluorescent articles comprising a screen layer and a layer containing a fluorescent coloring agent wherein the screen layer only permits a defined range of light transmission. European Patent Application No. 91311189.4 also discloses a retroreflective article comprising an ultraviolet screening layer and a color layer contained in a defined polymeric matrix. The article exhibits durable daylight fluorescence and resistance to degradation from exposure to sunlight.
Articles which exhibit enhanced fluorescence and color durability, even when exposed to ultraviolet radiation, are needed. In particular, fluorescent articles which retain their color and/or their fluorescent properties without requiring the use of protectant overlays are needed.